A critical problem that needs to be resolved in modern wireless communication is: how to further improve spectrum utilization and transmission reliability of a system. Multiple-input multiple-output (MIMO) is a mainstream technology in current wireless communication, and is adopted into many standards such as 802.11, 802.16, and 802.15. In a MIMO technology, multiple antennas are used at both a receive end and a transmit end to form a multi-antenna system, which can effectively increase a communication capacity and improve communication quality, and can meet a requirement of large-capacity high-quality communication, effectively improve spectrum utilization, and alleviate an increasingly urgent need for spectrum resources.
The MIMO technology has already been widely adopted in existing wireless local area network (WLAN) standards. The IEEE 802.11n standard supports a maximum of 4×4 MIMO transmission (a quantity of transmit antennas and a quantity of receive antennas are both four), while the IEEE 802.11ac standard supports a maximum of 8×8 MIMO transmission (a quantity of transmit antennas and a quantity of receive antennas are both eight). In addition, a data transmission frame in WLAN is divided into two parts: a preamble (Preamble) part and a data part. When receiving a signal, a receiver in a WLAN system first needs to adjust a power gain of the received signal, so that the signal enters an analog-to-digital converter (ADC) with appropriate power to be converted into a digital signal, and digital processing is further performed on the received signal. For adjusting power of the received signal, in an existing WLAN standard, two stages of automatic gain control (AGC) adjustment are performed on the received signal by respectively using a legacy short training field (L-STF) sequence, and a high-throughput short training field (HT-STF) sequence or a very-high-throughput short training field (VHT-STF) sequence (in the prior art, the HT-STF and the VHT-STF are collectively referred to as high-efficiency short training fields) in a preamble sequence. For details, refer to FIG. 1.
FIG. 1 is a schematic diagram of a data frame structure and AGC adjustment by a receiver in an existing WLAN standard. As shown in the figure, the receiver performs stage-1 (preliminary) AGC adjustment by using an L-STF sequence in a preamble sequence, and in the preliminary AGC adjustment, a received signal may be roughly adjusted to a dynamic range of an ADC. In addition, the receiver performs stage-2 (accurate) AGC adjustment by using a VHT-STF sequence in the preamble sequence (FIG. 1 shows a case in the 802.11ac standard, and in 802.11n, stage-2 (accurate) AGC adjustment is performed by using an HT-STF sequence), and in the accurate AGC adjustment, the received signal may be accurately adjusted to the dynamic range of the ADC.
If AGC adjustment is performed on a received signal in the foregoing manner, it is necessary to make power of an STF sequence part of the received signal match power of a data part of the signal as much as possible. If in the received signal, the power of the STF sequence is greater than the power of the data part, an entire power gain of the received signal will be set to be excessively high, and consequently, saturation or peak clipping of the data part of the signal is caused. If the power of the STF sequence in the received signal is less than the power of the data part, an entire power gain of the received signal will be set to be excessively small, and consequently, sampling precision of the data part of the signal is insufficient in the ADC. To prevent the two cases mentioned above from occurring and impacting entire receiving performance of a system, it is necessary to make the power of the STF sequence match the power of the data part.
When there are multiple transmit antennas at the transmit end of the system, transmit antennas send same STF sequences in the preamble sequence, and for the data part, mutually independent data streams may be sent. In this case, amplitude superposition occurs in the STF sequence, and consequently, the power of the STF sequence is severely deviated from the power of the data part. To avoid such a beamforming effect, a cyclic shift delay is introduced into the WLAN system. A larger CSD value indicates that the receiver may obtain better AGC adjustment performance. In addition, when a quantity of transmit antennas is larger, the system needs a larger CSD value to obtain better AGC performance.
Currently, applying a CSD value greater than 200 ns to an L-STF sequence affects a cross correlation of sequences, and consequently affects correct reception of an L-SIG sequence. Therefore, in the existing WLAN standards, for an L-STF/L-LTF/L-SIG part, a maximum CSD value of a preamble sequence is limited to be not greater than 200 ns.
In the prior art, a solution used in the IEEE 802.11n standard is that a CSD sequence of a maximum of 200 ns is used for a legacy part L-LTF sequence of a preamble sequence, where the IEEE 802.11n standard supports a maximum of four transmit antennas. CSD sequences used for different quantities of transmit antennas are shown in FIG. 2.
In addition, in the solution used in the IEEE 802.11n standard, an L-STF sequence is reused for an HT-STF. Differently, an OFDM symbol whose duration is 4 μs is used in the HT-STF part. CSD sequences used for the HT-STF part are shown in FIG. 3.
It may be learned from FIG. 3 that, to obtain better AGC performance, a maximum CSD value used for the HT-STF sequence in the IEEE 802.11n standard is increased to 600 ns.
A CSD value of the HT-STF in the prior art is increased correspondingly. However, a limitation of the solution is: the L-STF sequence is reused for the HT-STF sequence in the system, and in this way, there are five cycles in 4 μs of the HT-STF sequence, where duration of each cycle is 800 ns. In this way, applying a CSD of 800 ns to the sequence is equivalent to applying a CSD of 0 ns to the sequence because cycles in this time sequence coincide with each other. Therefore, in the prior art, a maximum CSD value possibly used for the sequence can be limited to be only 750 ns (a sampling interval of time domain signals in a bandwidth of 20 M is 50 ns).
Similar to the IEEE 802.11n standard, a solution used in the IEEE 802.11ac standard is that a CSD sequence of a maximum of 200 ns is used for a legacy part L-STF sequence in a preamble sequence; however, a maximum quantity of supported transmit antennas is increased to 8. For the legacy part L-STF in the preamble sequence, CSD sequences used for different quantities of transmit antennas are shown in FIG. 4.
Similar to the 11n, in the 11ac standard, an L-STF sequence is also reused for a VHT-STF, and an OFDM symbol whose duration is 4 μs is used in a VHT-STF part in the 11ac standard. CSD sequences used for the VHT-STF part are shown in FIG. 5.
It may be learned from FIG. 5 that, to obtain better AGC performance, a maximum CSD value for a VHT part of a preamble sequence and a data part is increased to a limit value: 750 ns.
Similar to the IEEE 802.11n standard, a limitation of the current solution in the IEEE 802.11ac standard is: the L-STF sequence is reused for the VHT-STF sequence in the system, and in this way, there are five cycles in 4 μs of the VHT-STF sequence, where duration of each cycle is 800 ns. Therefore, a maximum CSD value possibly used for the sequence also can be limited to only 750 ns (a sampling interval of time domain signals in a bandwidth of 20 M is 50 ns).
In conclusion, in the prior art, a cycle of a high-efficiency short training field sequence in a preamble sequence in a WLAN system is short, and consequently, a maximum CSD value that can be used is extremely limited. Eventually, AGC adjustment performance may be unsatisfactory.